1. Field of the Invention
The present invention relates to a secondary battery comprising a nonaqueous electrolyte and to a battery pack comprising the secondary battery.
2. Description of the Related Art
In recent years, the market of portable information apparatuses such as a portable telephone and a small personal computer is being rapidly expanded. With progress in the miniaturization and in the weight reduction of these portable information apparatuses, the power source of the apparatus is also required to be miniaturized and to be made lighter in weight. A lithium ion secondary battery having a high energy density is used in many of these portable information apparatuses, and a research on the lithium ion secondary battery is being conducted even nowadays. In recent years, the miniaturization is being promoted in various apparatuses such as the digital audio apparatus and the POS terminal in accordance with progress of the technology. When the apparatus is made portable by miniatuarization, a battery that permits eliminating the power source cord is required in place of the conventional AC power source. Under these circumstances, the field of use of the secondary battery is being expanded. It should also be noted that the improvement of the battery performance is always required in the information apparatus such as a personal computer and the portable telephone in which the secondary battery continued to be used in the past. A lead-acid battery, a nickel-cadmium secondary battery, a nickel-hydrogen secondary battery, etc. were used in the past as the secondary battery. However, these conventional secondary batteries were not satisfactory in respect of the requirements for the miniaturization and the weight reduction. On the other hand, a nonaqueous electrolyte secondary battery is small in size, low in weight and has a large capacity. Therefore, the nonaqueous electrolyte secondary battery has come to be used in the personal computer and the portable telephone noted above as well as in a digital camera, a video camera, etc.
The nonaqueous electrolyte secondary battery noted above includes a secondary battery comprising a positive electrode active material, a negative electrode active material, and an electrolyte. A lithium-containing cobalt composite oxide or a lithium-containing nickel composite oxide is used as the positive electrode active material. Also, a graphite series or a coke series carbon material is used as the negative electrode active material. Further, an organic solvent having a lithium salt such as LiPF6 or LiBF4 dissolved therein is used as the electrolysis solution. Each of the positive electrode and the negative electrode is used as sheets. The positive and negative electrodes are electronically insulated from each other because a separator holding the electrolyte is arranged between the positive electrode and the negative electrode. Further, the positive electrode, the negative electrode and the separator are housed in a container having any of various shapes to form a battery.
The electrolysis solution used in the nonaqueous electrolyte secondary battery described above contains a flammable organic solvent as a main component. Therefore, a chemical reaction that is not supposed to take place in the ordinary charge-discharge operation may occur, due to, for example, over-charging, which makes the secondary battery thermally unstable. In such a case, it is possible that the safety of the battery to be impaired by the combustion of the organic solvent.
It was studied in the past to change the composition of the electrolyte solution for overcoming the problem noted above. In the electrolyte solution based on the organic solvent, ethylene carbonate, diethyl carbonate, ethyl methyl carbonate or γ-butyrolactone etc. was used as the solvent. These solvents have a flash point of 152° C., 31° C., 24° C. and 98° C., respectively. It was attempted to improve the safety of the battery by using ethylene carbonate or γ-butyrolactone as the solvent, which have a relatively high flash point among the solvents noted above. However, an example is reported that the temperature within a vehicle exceeded 100° C. in summer, which indicates that the use of ethylene carbonate and/or γ-butyrolactone is not sufficient for improving the safety of the battery.
It is also attempted to improve the safety of the battery by adding a flame-retardant phosphoric ester to the organic solvent-based electrolyte solution. Trimethyl phosphate (TMP) is added in a large amount to the electrolyte solution to improve the safety but it causes the deterioration of the performance of the secondary battery. For example, it is described in JP-A No. 2001-160415 (KOKAI) that an allyl series phosphoric ester such as dimethyl allyl phosphate is added to the electrolyte solution in order to suppress the performance detrioration by large amount TMP addition. As a matter of fact, the patent document quoted above refers to an Example in which dimethyl allyl phosphate, TMP and a graphite series negative electrode active material are used together. On the other hand, it is described in JP-A No. 2002-203598 (KOKAI) that a lactone series organic solvent such as γ-butyrolactone brings about a reaction with a graphite series negative electrode active material and that, in order to suppress the particular reaction, an additive is added to the electrolysis solution. The additive noted above includes an allyl series phosphoric ester such as dimethyl allyl phosphate. It should be noted that, in each of the examples quoted above, dimethyl allyl phosphate is used as the allyl series phosphoric ester in order to suppress the reaction between the organic solvent such as TMP or a lactone series organic solvent and the negative electrode active material.
However, the nonaqueous electrolysis solution used in the lithium secondary battery described in each of JP-A No. 2001-160415 (KOKAI) and JP-A No. 2002-203598 (KOKAI) referred to above contains an organic solvent as an indispensable component and, thus, the electrolysis solution inevitably has the flash point. Such being the situation, it was impossible for the lithium secondary battery to secure a sufficient safety under the conditions involving various dangers such as the internal short-circuiting, the external short-circuiting, the flame spread from the outside and the compressive breakage of the battery.
As another attempt, there is a research in which an ionic liquid that assumes a liquid state under room temperature and doe not have the flash point is used to improve the safety of the battery. For example, a nonaqueous electrolyte secondary battery with the ionic liquid as the electrolyte is disclosed in JP-A No. 4-349365 (KOKAI) as a secondary battery with excellent safety. In the secondary battery disclosed in this patent document, a lithium metal oxide is used as the positive electrode active material. Also, a lithium metal, a lithium alloy or a carbonaceous material capable of intercalating or deintercalating lithium ions is used as the negative electrode active material. Further, an ionic liquid containing a lithium salt, aluminum halide, and a halogenated quaternary ammonium is used as the electrolyte.
Further, JP-A No. 11-86905 (KOKAI) discloses a nonaqueous electrolyte secondary battery having a composition explained below as a secondary battery excellent in safety and enhanced in the charge-discharge cycle life and in the discharge capacity. This nonaqueous electrolyte secondary battery comprises a positive electrode, a negative electrode containing a carbonaceous material intercalating or deintercalating lithium ions, and an ionic liquid used as an electrolyte and formed of quaternary ammonium ion, lithium ion and fluoride anion containing an element selected from the boron, phosphorus and sulfur.
However, the ionic liquid noted above reacts with the positive electrode or the negative electrode. As a result, the charge-discharge cycle life of the secondary battery is shortened.